Method and apparatus for reducing interference effects caused by microwave sources

ABSTRACT

A microwave appliance ( 10 ) designed to communicate with a plurality of communication devices ( 11,13 , &amp;  15 ) within a piconet ( 30 ) includes a transmitter ( 24 ) for communicating with the plurality of communication devices within the piconet; and a processor ( 22 ). The processor is programmed to communicate ( 102 ) to a master within the piconet about operating information of the microwave appliance, delay bias ( 104 ) to a microwave source within the microwave appliance for a predetermined time upon a user request for microwave appliance operation and then return ( 108 ) to a normal operating condition.

FIELD OF THE INVENTION

The present invention is directed to microwave interference, and moreparticularly to a means of reducing microwave interference among anetworked plurality of wireless communication devices.

BACKGROUND OF THE INVENTION

Devices using the emerging Personal Area Network (PAN) protocolsincluding Bluetooth HomeRF or 802.11 protocols occupy the same radiofrequency spectrum as do microwave ovens. Bursts of RF interference fromthe microwave oven occurs with a repetition rate equal to that of,thealternating current (ac) line frequency supplying the oven. In microwaveovens in particular, a magnetron is typically supplied by the mainsdirectly, via an autotransformer, with no supply filtering. Theresulting RF bursts occur only during one half of the ac cycle.Radiation from microwave ovens may be quite strong (in fact equal instrength to that of a Bluetooth device itself), and quite broadband,covering the entire 2.45 GHz ISM band used by Bluetooth devices. Thisbroadband interference covers all possible Bluetooth channels, so thatBluetooth's principal interference-avoidance strategy(frequency-hopping) is in this case ineffective. Both synchronous voice(SCO) and asynchronous data (ACL) Bluetooth communication links areaffected. During half of the ac cycle the SCO voice packets are simplylost, resulting in significantly reduced audio quality, while the ACLlinks suffer multiple NACKs and reduced data throughput as the links arelost and then reestablished each ac cycle.

A primary market for personal area networks and particularly Bluetoothis at-home use by consumers, as either a cordless telephone system or ahome wireless network connecting intelligent appliances. In either ofthese applications the loss of the personal area network while amicrowave oven is on is clearly unacceptable; similarly, operation ofthe microwave oven cannot be prohibited during use of the personal areanetwork. Thus, a solution is needed to allow microwave ovens andBluetooth networks to coexist.

A relevant FCC regulation for Bluetooth, Code of Federal Regulations,Title 47, Sec. 15.247, states in paragraph (h): “The incorporation ofintelligence within a frequency hopping spread spectrum system thatpermits the system to recognize other users within the spectrum band sothat it individually and independently chooses and adapts its hopsets toavoid hopping on occupied channels is permitted.”

Use of algorithms for detecting periodicity on desired signals such asbaud detection algorithms is well known. The present invention utilizesa baud detection algorithm in a novel way to identify interferencehaving a particular periodicity (50 or 60 Hz) indicative of microwaveoven interference. Conventional noise blankers, which typically blankreceiver audio output when a measured noise level exceeds a threshold,are also known in the art, but they differ from the present invention inthat they are essentially “real time” devices without memory, and do notdistinguish between interference occurring at different repetitionrates.

U.S. Pat. No. 5,838,741, (entitled “Communication Device and method forreducing effects of noise introduction by synchronizing data transfer toa received signal,” by Callaway, Ansari, Mock, Eaton, and Hayes, issuedNov. 17, 1998) describes an interference avoidance method in whichnoise-producing activities are timed to occur during periods of thedesired signal that are not sensitive to noise (e.g., symboltransitions). The present invention differs from U.S. Pat. No. 5,838,741in that, in the present invention, the desired system operation issynchronized to existing, externally generated, periodic noise.

U.S. Pat. No. 6,006,071 (entitled “RF Communication System Operable inthe Presence of a Repetitive Interference Source and Related Methods”)that not only appears to require forward error correction, but alsorequires the sending of data packets twice. '071 describes a one-waysystem for wireless speakers where no attempt is made to determine whichhalf-cycle is causing the interference. In essence, data packets aresent twice in synchrony with the AC cycle so that one of the datapackets will be in the clear half-cycle. '071 also uses AC sensingexternal to the oven/appliance, and so it is possible that the sensorand the oven are on different phases of the AC supply (most houses arefed with multi-phase AC power). In the present invention, current wouldbe sensed going to the magnetron by phase detector 14 in FIG. 1,eliminating this phase uncertainty. Additionally, in the presentinvention, any communication between the microwave oven and otherdevices in a piconet would be synchronous to power supplied to themagnetron, rather than the AC mains themselves as shown in '071.

U.S. Pat. No. 5,574,979 requires tracking an alternating currentassociated with a power main, whereas the present invention just detectsand tracks the periodic interference alone, and is thus suitable forportable battery powered devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an appliance with a microwave source in apersonal area network in accordance with the present invention.

FIG. 2 is a block diagram of a communication device capable of receivingand mitigating the effects of microwave interference in accordance withthe present invention.

FIG. 3 is a flow chart illustrating a method of mitigating microwaveinterference from a microwave appliance in accordance with the presentinvention.

FIG. 4 is a flow chart illustrating a method of operating acommunication device subject to microwave interference within acommunication network in accordance with the present invention.

FIG. 5 is another flow chart illustrating a method of operating acommunication device subject to microwave interference within acommunication network in accordance with the present invention.

DETAILED DESCRIPTION

The present invention discloses various methods of mitigating microwaveinterference among communication devices operating in a personal areanetwork, local area network, or piconet, wherein any of these termsshall hereinafter be referred to synonymously as piconet. In oneembodiment, the microwave source causing the interference is leftunaltered and the communication devices subject to the interferenceperform mitigation techniques in response. In another embodiment, themicrowave source proactively notifies the communication devices, givingthe communication devices an opportunity to operate in modes that wouldmitigate the interference. In yet another embodiment, the microwavesource and the communication devices within the network operatecooperatively to mitigate the interference effects. The presentinvention is described with respect to Bluetooth devices that detect thepresence of a periodic interferer (the oven) and take appropriatemitigating action. In the context of a Bluetooth piconet, a microwaveappliance acting as a slave can be used to communicate actively with theBluetooth piconet master, rather than just exist as a passive noisesource. In addition, the present invention could also be embodied havingthe microwave appliance acting as the piconet master instead of a slave.

In a first embodiment, a microwave appliance such as theBluetooth-equipped microwave oven 10 operating in a communicationnetwork 30 preferably includes the addition of specific functions sothat it may inform the master(s) (11, 13, or 15 or 24 when thetransceiver 24 acts as a master)) of local piconet(s) (30) of its statusas an interfering device as shown in FIG. 1. The masters, in turn,reconfigure their piconets to minimize the effects of the oveninterference on their operation.

Ideally, the microwave oven 10 comprises in part a conventionalmicrowave oven including a transformer 18 for converting an alternatingcurrent source to an appropriate voltage. The transformer 18 ispreferably coupled to a microwave source 12 which is typically amagnetron, but may also be a backward wave oscillator, a traveling wavetube, a klystron, or other microwave source. In addition to theconventional microwave oven components, the microwave oven 10 furthercomprises phase detector 14 preferably coupled in series with themicrowave source 12 to directly measure the phase of the current appliedto the microwave source 12. Alternatively, phase detector 14 may becoupled in shunt with the microwave source 12 to directly measure thephase of the voltage applied to the microwave source 12. The phasedetector 14 is preferably coupled to a processor or controller 22 and atransceiver 24 such as a Bluetooth transceiver. The transceiver 24preferably communicates within a piconet of Bluetooth enabledcommunication devices 11, 13, and 15 as shown. It should be understood,that elements 14, 22, and 24 are shown as additional items to aconventional microwave oven since certain embodiments of the presentinvention could also operate and mitigate the interference fromconventional ovens without these modifications.

Specifically (and assuming the appliance or oven 10 itself is not thepiconet master and that device 15 is the master for piconet 30), when auser turns the appliance on, bias to the microwave source 12 is delayeda few milliseconds while the appliance contacts the master device 15 (orthe “master”) to report that it is about to start. In this report, theoven 10 includes operating information such as the length of time theoven is programmed to run, when it will start (preferably measured inBluetooth slot times, e.g., 32 slot times [10 milliseconds] from now),the phase of the ac cycle, etc. The master then has sufficient advancewarning and description of the impending interference to takeappropriate countermeasures, such as increasing transmit power,synchronizing the piconet operation with the “quiet” half of the accycle (as opposed to the “noisy” half of the ac cycle), placing unneededslaves in lower-power Bluetooth modes (sniff, hold, park) for theduration of the interference, etc. If the oven is itself the piconetmaster, these countermeasures are taken using the same operatinginformation. The oven can even communicate a new status to the piconetmaster such as when the oven is turned off prematurely (due to the useropening the oven door, for example). This fact is reported to thepiconet master, which may return the piconet to its previousconfiguration. Note that, by transmitting and receiving during the“quiet” half of the ac cycle, the microwave oven can remain an activeparticipant of the Bluetooth piconet even while the oven is operating.

In another embodiment of the present invention, a method uses aperiodicity-determining algorithm, e.g., the Skunk baud detectionalgorithm, on a channel metric to detect the existence of a particularperiodic interferer, to which the Bluetooth device adapts to maximizethe channel quality of service. Said more simply, an algorithmpreferably detects the presence of a 50 or a 60 Hz interfering noisefrom a microwave oven. The Bluetooth device then synchronizes itsoperation to the noise, operating only during the half-cycle periodswhen the interfering signal is silent. A communication device 40 capableof utilizing the algorithm described above and as will be described infurther detail below is shown in FIG. 2. The communication device 40preferably comprises a receiver 42 coupled to a phase detector 46 via acorresponding demodulator 44, and received signal strength indicator(RSSI) 48. The device 40 also preferably comprises a transmitter 54coupled to a corresponding modulator 52. The transmitter and receiver aswell as the modulator and demodulator are preferably coupled to aprocessor 50 which controls many of the operating functions of thecommunication device 40. Additionally, the processor 50 may also becoupled to selector switches 56, alerts 58, displays 60 and powerswitches 51 as is well known in the art.

If one considers the 50 Hz ac mains found in Europe, a microwave ovennoise burst may last 10 ms, followed by a 10 ms silent period. Bluetoothoperates with a slot rate of 1600 slots/s (625 us/slot); in a 10 msperiod there are 10/0.625=16 slots. With this number of slots per ovennoise burst, the Bluetooth device may employ one or more of severalchannel quality metrics as an input to the periodicity-determiningalgorithm. It may monitor the number of corrupted packets (or NACKs)received, or it may track only corrupted packets received with aReceived Signal Strength Indicator (RSSI) value above some threshold.With some loss in battery life, it may even track RSSI levels in betweenpiconet transmission times, when no desired signals should be present.

Whatever metric is used, it is checked for the presence of 50 Hzperiodicity. What happens when it is found depends on whether theBluetooth device is the master of the piconet, or a slave. If theBluetooth device is the master, it reorganizes the piconet; itreschedules piconet traffic such that communication is not scheduledduring the noisy half-cycle. Inactive slaves may be placed in Hold,Sniff, or Park mode, improving their battery life by reducing their dutycycle. If the Bluetooth device is a slave, it reports the phase of thenoise to the master; the master then knows not to attempt contact withthe slave during the noisy periods, and may in fact place the slave inHold, Sniff, or Park mode, to reduce its activity during the noisyperiods. If the transmitting Bluetooth device is equipped with powercontrol, another interference-mitigating strategy is to maximizetransmit power during the noise bursts.

While it may seem that many Bluetooth slots may get blocked beforedetecting the periodic noise (the algorithm may require a cycle or twoof 50 Hz noise to detect it reliably—at 32 Bluetooth slots per cycle),microwave ovens are generally on for minutes at a time, and there are96,000 Bluetooth slots in a minute. There is, therefore, much to begained by mitigating microwave oven interference.

Referring to FIG. 3, a flow chart illustrates a method 100 of microwavesource control to limit signal interference among a plurality of devicescommunicating in a piconet operating as master and slaves within apredetermined proximity of a microwave source. In step 102, the methodwould delay applying bias to the microwave source within the appliancefor a predetermined time. Next at step 104, a microwave appliancepreferably communicates with a master within the piconet about operatinginformation of the microwave source upon a request for activation of themicrowave source. The operating information can comprise one or more ofthe following: a length of time or times the microwave source isprogrammed to be active; when the microwave source will begin operation;or the phase of an alternating current cycle the microwave source isoperating on. It should be understood within contemplation of thepresent invention that the predetermined time can simply be the delayinherent in activating the microwave source (e.g., time required forfilament heating). The predetermined time should also be understood asbeing a static delay of a fixed duration or a dynamic delay based upontermination of desired functions such as completing communicationsbetween the appliance and the master.

At this point, the master can direct the appliance into a Hold, Sniff,or Park mode without receiving a request from the appliance. Next, themethod at decision block 106 determines whether communication with themaster is complete. In effect, this allows the microwave source to delaybias to the microwave source until the microwave source completescommunication to the master about its operating information. Once thecommunication is complete, the method returns the microwave source to anormal operating condition at step 108. In the interim, if the microwaveappliance has a new status at decision block 110, the method furthercomprises the step 112 of communicating with the master of such newstatus. For example, such new status could be if the appliance is turnedoff prematurely. If there is no new status at decision block 110, theappliance returns to normal operation as shown.

If the appliance operates as a piconet slave, then the method furthercomprises the alternative step of requesting to be placed in a Hold,Sniff, or Park mode while the microwave source is operating during anoisy half-cycle as shown in step 105. Subsequently, the master respondsby placing such piconet slave in the appropriate mode.

If the appliance operates as a master of the piconet, then the methodmay further comprise the alternate steps 107 of re-organizing theplurality of devices communicating on the piconet by the appliance and109 of rescheduling communication among the plurality of devices suchthat communication among the plurality of devices is not scheduledduring a noisy half-cycle of operation of the appliance and/or the step111 of placing inactive slaves among the plurality of devices into aHold, Sniff, or Park mode. This can also mean that the applianceoperating as a master within the piconet would also be placed in a Hold,Sniff, or Park mode while the microwave source is in operation.

FIG. 4 is a flow chart illustrating a method 80 of operating acommunication device within a piconet that is subject to interferencefrom signals emitted by a microwave oven, wherein the oven is notspecifically designed to “communicate” with the communication devices onthe piconet. The method 80 preferably comprises the step 82 of receivingthe signals emitted from the microwave oven in operation at thecommunication device and the step of detecting periodicity or a quiethalf cycle from the signals emitted from the microwave oven as shown indecision block 84. Detecting demodulated noise, corrupt packets, RSSI,or some combination thereof could be used as an input to the periodicitydetection algorithm used at decision block 84. Typically, the step ofdetecting comprises the step of detecting a 50 or 60 Hertz periodicnoise. Alternatively, the detection step could also comprise (aspreviously mentioned) the steps of detecting a number of corruptedpackets, particularly when a received signal strength measurement isabove a predetermined threshold and when no desired signals are found.If a “noisy” half cycle or a “quiet” half cycle is not detected, thenthe master at step 86 may utilize other interference mitigationtechniques or otherwise return to normal operation. If periodic noise(i.e., containing a “noisy” or “quiet” half cycle) is detected, then themaster goes on at step 88 to counter the interference. Such counteringcould include as previously discussed increasing the transmit power ofthe communication device; synchronizing the piconet operation with thequiet half cycle; operating the communication device only during thequiet half cycle; or placing unneeded slaves in the piconet in alow-power mode during a period of quiet half cycle detection.

FIG. 5 is another flow chart illustrating a method 70 of operating acommunication device within a piconet that is subject to interferencefrom signals emitted by a microwave oven designed specifically tocommunicate with the communication device. Instead of receivingmicrowave interference and detecting such interference as in method 80of FIG. 4, the communication device within the piconet receivesinformation from the microwave oven concerning the microwave oven'soperating parameters at step 72 and then adjusts the operation of thecommunication device at step 74 based on the information received fromthe microwave oven to mitigate such interference effects. Again, suchoperating parameters can comprise, among other things, a length of timethe microwave oven is programmed to run, when the microwave oven willbegin operation, or the phase of an alternating current cycle themicrowave oven is operating on. As previously described, the adjustmentscould also comprise among other things increasing the transmit power ofthe communication device, synchronizing the piconet operation with thequiet half cycle, operating the communication device only during thequiet half cycle, or placing unneeded slaves in the piconet in alow-power mode during a period of quiet half cycle detection.

The above description is intended by way of example only and is notintended to limit the present invention in any way except as set forthin the following claims.

What is claimed is:
 1. A method of microwave source control within anappliance to limit signal interference among a plurality of devicescommunicating in a piconet, comprising the steps of: delaying bias to amicrowave source within the appliance for a predetermined time; andcommunicating with a member of the piconet about operating informationof the microwave source upon a request for activation of the microwavesource.
 2. The method of claim 1, wherein the plurality of devicescommunicating in the piconet operate as master and slaves.
 3. The methodof claim 1, wherein the method further comprises the step of returningthe microwave source to a normal operating condition.
 4. The method ofclaim 1, wherein the step of delaying bias to the microwave sourcecomprises delaying bias to the microwave source until the microwavesource completes communication to the master about its operatinginformation.
 5. The method of claim 1, wherein the step of communicatingfurther comprises the step of communicating information selected fromthe group comprising a length of time or times the microwave source isprogrammed to be active, when the microwave source will begin operation,or the phase of an alternating current cycle the microwave source isoperating on.
 6. The method of claim 1, wherein the method furthercomprises the step of communicating with the master of a new status ifthe appliance is turned off prematurely.
 7. The method of claim 1,wherein if the appliance operates as a master of the piconet, then themethod further comprises the step of re-organizing the plurality ofdevices communicating on the piconet by the appliance.
 8. The method ofclaim 7, wherein the step of re-organizing comprises the step ofrescheduling communication among the plurality of devices such thatcommunication among the plurality of devices is not scheduled during anoisy half-cycle of operation of the appliance.
 9. The method of claim7, wherein the step of re-organizing comprises the step of placinginactive slaves among the plurality of devices into a hold, park, orsniff mode.
 10. The method of claim 1, wherein if the appliance operatesas a slave of the piconet, then the method further comprises the step ofplacing the appliance in a hold, park, or sniff mode while the microwavesource is in operation.
 11. The method of claim 1, wherein if anappliance having the microwave source operates as a slave of thepiconet, then the method further comprises the step of making a requestto the master for a change in mode by the appliance and subsequentlyplacing the appliance in a hold, park or sniff mode while the microwavesource is operating during a noisy half-cycle.
 12. The method of claim1, wherein the step of communicating operating information of themicrowave source comprises the step of detecting phase informationapplied to a magnetron directly acting as the microwave source.
 13. Amethod of operating a communication device within a piconet that issubject to interference from signals emitted by a microwave ovendesigned to communicate with the communication device, comprising thesteps of: receiving information from the microwave oven concerning themicrowave oven's operating parameters; and adjusting the operation ofthe communication device based on the information received from themicrowave oven to mitigate interference effects.
 14. The method of claim13, wherein the step of receiving information from the microwave ovencomprises the step of receiving information selected from the groupcomprising a length of time the microwave oven is programmed to run,when the microwave oven will begin operation, or the phase of analternating current cycle the microwave oven is operating on.
 15. Themethod of claim 13, wherein the step of adjusting the operation of thecommunication device is selected from the group of steps comprisingincreasing the transmit power of the communication device, synchronizingthe piconet operation with the quiet half cycle, operating thecommunication device only during the quiet half cycle, or placingunneeded devices in the piconet in a low-power mode during a period ofquiet half cycle detection.
 16. A microwave appliance designed tocommunicate with a plurality of communication devices within a piconet,comprises: a transmitter for communicating with the plurality ofcommunication devices within the piconet; and a processor programmed to:communicate to a master within the piconet about operating informationof the microwave appliance; delay bias to a microwave source within themicrowave appliance for a predetermined time upon a user request formicrowave appliance operation; and return the microwave appliance to anormal operating condition.
 17. The microwave appliance of claim 16,wherein the microwave source is selected from the group consisting of amagnetron, a backward wave oscillator, a traveling wave tube, or aklystron.
 18. The microwave appliance of claim 16, wherein the microwaveappliance further comprises a receiver coupled to the processor.